c-kit is expressed by primitive human hematopoietic cells that give rise to colony-forming cells in stroma-dependent or cytokine-supplemented culture.

Using monoclonal antibody (MAB) YB5.B8, we have examined the expression of the c-kit protein, the receptor for the hematopoietic cytokine stem cell factor (SCF), on primitive hematopoietic cells. Bone marrow mononuclear cells (BMMNC) enriched for immature cells by differential agglutination using the lectin soybean agglutinin (SBA) were subjected to multiparameter fluorescence activated cell sorting (FACS) based on light-scattering properties, the expression of the c-kit protein and the CD34 antigen, and the retention of the vital fluorescent dye, Rhodamine 123 (Rh123). Sorted populations were assayed for their content of directly clonogenic progenitor cells (colony-forming units-granulocyte/macrophage [CFU-GM], burst-forming units-erythroid [BFU-E], and multipotential colony-forming units [CFU-Mix]) and for the presence of more primitive progenitor cells ("pre-CFU"). The latter were assayed by (1) their ability to initiate and sustain hematopoiesis in a standard stromal cell-dependent culture system and (2) their capacity for de novo generation of clonogenic progenitors in response to a combination of six recombinant hematopoietic cytokines in a stroma-independent suspension culture assay. A mean of 76% of CD34+ cells were found to coexpress c-kit. The majority of directly clonogenic cells (98% of CFU-GM, 98% of CFU-Mix, and 85% of BFU-E) were found in the CD34+c-kit+ fraction. Similarly, all pre-CFU were recovered in the CD34+c-kit+Rh123dull fraction, irrespective of whether the cells were maintained on marrow stromal cells or in cytokine-supplemented liquid culture. A mean of 87% (range 70-100%) of the CD34+Rh123dull cells also expressed c-kit. Since SCF has been reported to act as a growth factor for early lymphoid cells as well as myeloid cells, we looked for coexpression of c-kit and early lymphoid markers in the CD34+ population by multiparameter flow cytometry. Coexpression of c-kit on a minority of cells with markers of B or T lineages was observed. The majority of early lymphoid cells, however, appeared to lack c-kit expression. This was confirmed by the finding that only 4% of c-kit+CD34+ cells showed terminal deoxynucleotidyl transferase (TdT) activity, compared with 25% of the c-kit-CD34+ cells.